In sensor elements that operate according to the limiting current principle, the limiting diffusion current is measured at a constant voltage applied to the two electrodes of the sensor element. This limiting diffusion current in an exhaust gas arising through combustion processes is dependent on the oxygen concentration as long as the diffusion of the gas at the so-called pump electrode determines the speed of the ongoing reaction. On the basis of a simplified and cost-effective production method, in recent years the manufacture of sensor elements using ceramic-foil and silk-screen technology has proven in practice to be advantageous. In a simple and efficient manner, planar sensor elements, based on wafer or foil-shaped oxygen-ion-conductive solid electrolytes, composed, e.g., of stabilized zirconium dioxide, can be manufactured that are coated on both sides having an inner and an outer pump electrode, respectively, and having the associated printed circuit trace. The inner pump electrode, in this context, is located in the edge area of the diffusion channel through which the measuring gas is fed. In the diffusion channel, a diffusion barrier, filled with a porous material, is formed constituting the gas diffusion resistance.
German Patent No. 35 43 759 describes a sensor element that includes a pump cell and a sensor cell, which are arranged in coating layers that are on top of each other. The sensor elements of this type are also designated as broadband (wideband) sensors, since they can detect the oxygen concentration of fuel/air mixtures ranging from lean to rich. The inner pump electrode of the pump cell and the measuring electrode of the sensor cell, in this context, are arranged opposite each other in a common measuring gas chamber, which at the same time forms the diffusion channel. The diffusion barrier is located in the diffusion channel upstream of the inner pump electrode and the measuring electrode in the direction of diffusion. A gas entry hole is led through the solid electrolyte foils on top thereof and through the layer thickness of the diffusion barrier, so that the inner cylinder wall of the diffusion barrier is part of the gas entry hole.
The manufacture of the diffusion barrier in the aforementioned sensor elements takes place such that a circular silk-screen layer is applied onto the corresponding solid electrolyte foil upstream of the electrodes using a silk-screen paste made, e.g., of ZrO2 and mixed with a pore-forming material. In the center of this silk-screen layer, once all the solid electrolyte foils have been laminated together, the gas entry hole is bored, penetrating at least the entire diffusion barrier. Upon sintering the solid electrolyte foils that have been laminated together, the porous diffusion barrier is then formed along with the hollow measuring gas chamber positioned upstream of the diffusion barrier.
In generating the gas entry hole, in the event of faulty boring parameters (speed, wear in the boring tool), it comes about that the material of the solid electrolyte foil plugs the pores in the inner cylinder wall of the diffusion barrier. This leads to a reduction of the gas entry cross-section after sintering, which ultimately means a large dispersion of the diffusion resistance. In addition, the disadvantage arises that the bored gas entry hole can deviate from the midpoint of the circular silk-screen layer of the diffusion barrier. This deviation leads to a shortening of the diffusion distance of the diffusion barrier and thus to a further alteration of the diffusion resistance. Furthermore, contamination, building up as a rule on the entry surface of the diffusion barrier, leads to a change in the sensor characteristic curve.
The sensor element of the present invention has the advantage that, when the gas entry hole is bored, the material of the solid electrolyte foils cannot clog the pores of the inner cylinder wall of the diffusion barrier. As a result, the diffusion resistance of the diffusion barrier is not impaired. In addition, a deviation of the centering of the gas entry hole only exerts an influence on the diffusion resistance of the diffusion barrier if the centering exceeds the difference between the boring radius and the inner radius of the diffusion barrier. Furthermore, as a result of the set-back inner wall of the diffusion barrier, the latter is shielded against contamination during extended engine use.
The method of the present invention has the advantage that as a result of the cavity-creating material, it is possible to produce a more defined inner diameter of the diffusion barrier, the cavity-creating material evaporating during the sintering of the sensor element and creating an inner chamber upstream of the diffusion hole.
It has proved to be advantageous to dispose the diffusion barrier so as to be set back roughly 0.1 to 0.3 mm from the wall of the gas entry hole. An advantageous refinement of the method involves pressing the inner chamber upstream of the diffusion barrier together with the cavity of the measuring gas chamber. Due to the shorter diffusion distance, a more planar diffusion barrier can also be used, that can be pressed in fewer silk-screen steps so as not to form cracks.